Various schemes for communicating information by modulating a carrier exist. Typical examples of such schemes are frequency shift keying (FSK), phase shift keying (PSK), and amplitude shift keying (ASK).
In phase shift keying, phases (0, pi) of a carrier are transmitted in correspondence with input data (0 or 1). Given two phases, one data bit may be transmitted per phase shift. With other phase shift keying schemes, more than one bit per shift may be transmitted. For example, in quadrature phase shift keying (QPSK), two bits successively input are regarded as one symbol, and phases (pi/4, 3pi/4, 5pi/4, 7pi/4) of a corresponding carrier are transmitted.
A quadrature phase shift keying hierarchy in which important data and unimportant data are differentiated from each other is disclosed in Published Unexamined Japanese Patent Application No. 5-276211.
In amplitude shift keying, the amplitude A of a carrier is modulated to carry, for example, two values (A/2, A) in correspondence with input data (0 or 1). The carrier modulated by amplitude shift keying is received by a receiver. The amplitude level of the received carrier is detected, and the input data corresponding to the detected amplitude level is reproduced. In this simple example, 1 is obtained as the input data when the amplitude level of the received wave is A, and 0 is obtained when the amplitude level is A/2. In this kind of amplitude shift keying, the rate of data transmission may be increased by increasing the number of different amplitude levels, i.e., increasing the number by which the amplitude A is divided.
Amplitude shift keying in wireless communication, however, can be easily affected by propagation path disturbances such as disturbances due to weather conditions such as rain or clouds. Fading may occur such that the received wave amplitude fluctuates in a short cycle. The amplitude of the carrier consequently fluctuates unpredictably, and the bit error rate thereby increases. In the above-described example, if a carrier with amplitude A, which for example corresponds to input data 1, undergoes a disturbance that results in the amplitude being halved to A/2, incorrect input data 0 is reproduced. For this reason, phase shift keying, which is advantageous in terms of noise resistance, is frequently used instead of amplitude shift keying.
On the other hand, cable broadcasting such as CATV is free from the above-described attenuation problem, and amplitude shift keying is effective in this environment. For cable broadcasting, therefore, keying schemes that combine amplitude shift keying and phase shift keying are being used. Such a keying scheme has been introduced, for example, in Published Unexamined Japanese Patent Application No. 63-175542.
For example, binary phase shift keying and binary amplitude shift keying may be performed on each of two polarized carrier waves, in this case horizontally and vertically polarized waves that are orthogonal to each other. In this case, the phase shift keying and amplitude shift keying together enable transmission of 2×2=4 bits of data per polarization. Accordingly, 4×4=16 bits of data per baud can be transmitted on the entire carrier. Since phase shift keying is combined with amplitude shift keying, the amplitude of each polarized wave is determined by the amplitude of the envelope of the polarized wave.
As described above, if amplitude shift keying and phase shift keying are performed in combination on each of two independently polarized waves, a multiplicity of information bits can be transmitted in one frequency band. However, this keying scheme is not effective in wireless communication, because the above-described attenuation of carrier amplitude by propagation-path disturbances cannot be avoided. That is, the influence of a disturbance may fluctuate with time, thereby causing the absolute value of the amplitude of the received carrier also to fluctuate with time, resulting in failure to reliably reproduce transmitted data.
Therefore, an object of the present invention is to provide a wireless communication method which extends digital transmission capabilities so that amplitude shift keying may be applied to two differently polarized waves without suffering the problem of excessive error rates caused by propagation-path disturbances acting on carrier signal amplitudes.